1. Field of the Invention
The present invention relates to a control system for a hybrid vehicle, and in particular, relates to a control system for a hybrid vehicle capable of improving the fuel consumption efficiency.
2. Description of the Related Art
Conventionally, hybrid vehicles, which carry motors as power sources for driving in addition to engines, are known. Hybrid vehicles are classified into serial hybrid vehicles and parallel hybrid vehicles. In parallel hybrid vehicles, the motor connected to the engine assists the rotation of the drive shaft of the engine while charging a power storage unit using the motor as a generator.
In parallel hybrid vehicles, the motor assists the engine at the time of acceleration and a power storage unit such as a battery is charged by deceleration regeneration during deceleration, in order to maintain sufficient electrical energy in the power storage unit such as a batteries (hereinafter, called the remaining charge, or the “state of charge”) to meet the driver's demands. Specifically, since the engine and the motor are connected in series in the parallel hybrid vehicle, the driving system of the parallel hybrid vehicle can be constituted by a simple and light weight structure and this driving system is applicable to a variety of types of vehicles.
There are several types of the parallel hybrid vehicles, one of which is a type that is provided with a clutch between the engine and the motor in order to avoid the influence of the engine friction (the engine brake) during deceleration regeneration (see, for example, Japanese Patent application, First Publication No. 2000-97068), and another type is one wherein the engine, the motor, and the transmission are connected in series in order to provide an extremely simple driving system.
However, the driving system having a clutch between the engine and the motor has a structure that has a problem in that the applicability of this driving system to a variety of vehicles is restricted because of the complicated structure of this system and because the transmission efficiency of the power transmission system of this driving system is degraded even when the vehicle is running.
The driving system in which the engine, the motor, and the transmission are connected in series has a problem in that the amount of regeneration energy is reduced because of the aforementioned engine friction so that the assist amount by the motor is restricted.
In the latter driving system, in order to reduce engine friction during deceleration, the amount of regeneration may be increased by reducing the pumping loss drastically through the use of an electronically controlled throttle mechanism for fully opening the intake and the exhaust valves during deceleration. However, when the intake and the exhaust valves are fully opened as described above, introduction of fresh air into the exhaust system during deceleration causes lowering of the temperatures of the catalyst and the A/F (air fuel ratio) sensor, which results in causing a problem in that the optimum control of the exhaust gas will be degraded.